From BlenderWiki

Animation Editors

Animation is a big task, often involving repetitive work. This is why many different techniques have been developed over the years, to hasten, and reduce the workload facing animators.

To manage all these features, Blender has three Animation editors (three window types):

The Ipo Curve editor

This is the lowest-level editor, where you use one curve to control each animated property/setting.

The Ipo Curve Editor: used to setup interpolation curves

The Action editor

This editor is somewhat similar to the Ipo one – it gives a less-precise, more general view of an object’s animation Unlike the IPO editor, it allows multiple bones/objects to be edited in the same instance.

The Action Editor: used to setup an action

The NLA editor

The idea behind this high-level editor is borrowed from the non-linear video editors: each element represents an action, which you can move, duplicate, shrink/pull (i.e. fasten/slow down), etc., to your liking. If IPO keys are like mesh faces, an 'NLA action' is an object encapsulating those mesh faces, with a modifier.

The NLA Editor: used to edit many actions

The important thing to understand is that each editor is “built upon” the previous one:

• Ipo curves...
• ...are used by actions...
• ...which are used by the NLA.

Note also that once you have animated something (the position of an object, the diffuse color of a material, the mapping of a texture, etc), its Ipo curve always override all editing you might do “by hand” on the animated property(-ties), unless you “register” this editing in the relevant Ipo curve(s) by inserting keyframes (more on this later).